Chapter 15:Fundamentals of Metal Forming

DeGarmo’s Materials and Processes in Manufacturing

15.1 Introduction

Deformation processes have been designed to exploit the plasticity of engineering materials

Plasticity is the ability of a material to flow as a solid without deterioration of properties

Deformation processes require a large amount of force

Processes include bulk flow, simple shearing, or compound bending

States of Stress

15.2 Forming Processes: Independent Variables Forming processes consist of independent and

dependent variables Independent variables are the aspects of the

processes that the engineer or operator has direct control Starting material Starting geometry of the workpiece Tool or die geometry Lubrication Starting temperature Speed of operation Amount of deformation

Forming Operations

Forming Operations

Forming Operations

15.3 Dependent Variables

Dependent variables are those that are determined by the independent variable selection Force or power requirements Material properties of the product Exit or final temperature Surface finish and precision Nature of the material flow

15.4 Independent-Dependent Relationships Independent variables- control is direct and

immediate Dependent variables- control is entirely

indirect Determined by the process If a dependent variable needs to be controlled, the

designer must select the proper independent variable that changes the dependent variable

Independent-Dependent Relationships Information on the

interdependence of independent and dependent variables can be learned in three ways Experience Experiment Process modeling

Figure 15-1 Schematic representation of a metalforming system showing independent variables, dependent variables, and the various means of linking the two.

15.5 Process Modeling

Simulations are created using finite element modeling

Models can predict how a material will respond to a rolling process, fill a forging die, flow through an extrusion die, or solidify in a casting

Heat treatments can be simulation Costly trial and error development cycles can

be eliminated

15.6 General Parameters

Material being deformed must be characterized Strength or resistance for deformation Conditions at different temperatures Formability limits Reaction to lubricants

Speed of deformation and its effects Speed-sensitive materials- more energy is

required to produce the same results

15.7 Friction and Lubrication Under Metalworking Conditions High forces and pressures are required to deform a

material For some processes, 50% of the energy is spent in

overcoming friction Changes in lubrication can alter material flow, create

or eliminate defects, alter surface finish and dimensional precision, and modify product properties

Production rates, tool design, tool wear, and process optimization depend on the ability to determine and control friction

Friction Conditions

Metalforming friction differs from the friction encountered in mechanical devices

For light, elastic loads, friction is proportional to the applied pressure μ is the coefficient of friction

At high pressures, friction is related to the strength of the weaker material

Figure 15-2 The effect of contact pressure on the frictional resistance between two surfaces.

Friction

Friction is resistance to sliding along an interface

Resistance can be attributed to: Abrasion Adhesion

Resistance is proportional to the strength of the weaker material and the contact area

Surface Deterioration

Surface wear is related to friction Wear on the workpiece is not objectionable,

but wear on the tooling is Tooling wear is economically costly and can

impact dimensional precision Tolerance control can be lost Tool wear can impact the surface finish

Lubrication

Key to success in many metalforming operations

Primarily selected to reduce friction and tool wear, but may be used as a thermal barrier, coolant, or corrosion retardant

Other factors Ease of removal, lack of toxicity, odor,

flammability, reactivity, temperature, velocity, wetting characteristics

15.8 Temperature Concerns

Workpiece temperature can be one of the most important process variables

In general, an increase in temperature is related to a decrease in strength, increase in ductility, and decrease in the rate of strain hardening

Hot working Cold working Warm working

Hot Working

Plastic deformation of metals at a temperature above the recrystallization temperature

Temperature varies greatly with material Recrystallization removes the effects of strain

hardening Hot working may produce undesirable

reactions from the metal and its surroundings

Structure and Property Modification by Hot Working The size of grains upon cooling is not

typically uniform Undesirable grain shapes can be common

(such as columnar grains) Recrystallization is followed by:

grain growth additional deformation and recrystallization drop in temperature that will terminate diffusion

and freeze the recrystallized structure

Hot Working

Engineering properties can be improved through reorienting inclusion or impurities

During plastic deformation, impurities tend to flow along with the base metal or fraction into rows of fragments

Figure 15-4 Etched surface of a sectioned crankshaft showing the grain flow pattern that gives forged parts enhanced strength, fracture resistance, and fatigue life. (Courtesy of the Forging Industry Association, Cleveland, OH)

Figure 15-3 Cross section of a 4-in.-diameter case copper bar polished and etched to show the as-cast grain structure.

Temperature Variations in Hot Working Success or failure of a hot

deformation process often depends on the ability to control temperatures

Over 90% of the energy imparted to a deforming workpiece is converted to heat

Nonuniform temperatures may be produced and may result in cracking

Thin sections cool faster than thick sections

Figure 15-5 Schematic comparison of the grain flow in a machined thread (a) and a rolled thread (b). The rolling operation further deforms the axial structure produced by the previous wire- or rod-forming operations, while machining simply cuts through it.

Cold Working

Plastic deformation below the recrystallization temperature

Advantages as compared to hot working No heating required Better surface finish Superior dimensional control Better reproducibility Strength, fatigue, and wear are improved Directional properties can be imparted Contamination is minimized

Disadvantages of Cold Working Higher forces are required to initiate and complete

the deformation Heavier and more powerful equipment and stronger

tooling are required Less ductility is available Metal surfaces must be clean and scale-free Intermediate anneals may be required Imparted directional properties can be detrimental Undesirable residual stresses may be produced

Metal Properties and Cold Working Two features that are significant in selecting a material for

cold working are Magnitude of the yield-point stress Extent of the strain region from yield stress to fracture

Springback should also be considered when selecting a material

Figure 15-6 Use of true stress-true strain diagram to assess the suitability of two metals for cold working.

Initial and Final Properties in a Cold-Working Process Quality of the starting

material is important to the success or failure of the cold-working process

The starting material should be clean and free of oxide or scale that might cause abrasion to the dies or rolls

Figure 15-7 (Below) Stress-strain curve for a low-carbon steel showing the commonly observed yield-point runout; (Right) Luders bands or stretcher strains that form when this material is stretched to an amount less than the yield-point runout.

Additional Effects of Cold Working Annealing heat treatments

may be performed prior or at intermediate intervals to cold working

Heat treatments allows additional cold working and deformation processes

Cold working produces a structure where properties vary with direction, anisotropy

Figure 15-8 Mechanical properties of pure copper as a function of the amount of cold work (expressed in percent).

Warm Forming

Deformations produced at temperatures intermediate to cold and hot working

Advantages Reduced loads on the tooling and equipment Increased material ductility Possible reduction in the number of anneals Less scaling and decarburization Better dimensional precision and smoother

surfaces than hot working Used for processes such as forging and extrusion

Warm Forming

Figure 15-9 Increasing the temperature increases the formability of aluminum, decreasing the strength and increasing the ductility. (Adapted from data provided by Interlaken Technology Corporation, Chaska, MN)

Isothermal Forming

Deformation that occurs under constant temperature

Dies and tooling are heated to the same temperature as the workpiece

Eliminates cracking from nonuniform surface temperatures

Inert atmospheres may be used

Figure 15-10 Yield strength of various materials (as indicated by pressure required to forge a standard specimen) as a function of temperature. Materials with steep curves may require isothermal forming. (From “A Study of Forging Variables,” ML-TDR-64-95, March 1964; courtesy of Battelle Columbus Laboratories, Columbus, OH.)